fix OFF_MAX redefinition and a potential overflow

[acmepjz]

This addresses first two points of issue #281.

• Since OFF_MAX is not used in the code at all, I just remove it.
• SZ_MAX is used only at one place in the code, and 1024 * 1024 is always smaller than it, so it seems redundant.
• 16 * size could overflow, which makes the MIN returns 0 which is unwanted. I've changed the code such that if that happens, it also returns 1024 * 1024.